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I'm using a '49mm-52mm Step-Up Ring' and a '52mm Macro Reverse Adapter Ring for Pentax K' to reverse mount my SMC PENTAX-A 1:2 50mm lens which has a 49mm filter thread. It's working great and I don't see any disadvantage. Of course it would be cheaper and simpler to use a '49mm Macro Reverse Adapter' but I was unable to find one in the local store (many years ago).